Crude streams for the commercial production of olefins contain various compounds as impurities. Acetylenic and diene impurities need to be removed from the streams to produce acceptable quality olefin products. To produce olefins such as ethylene, propylene, butene, pentene and the like, acetylenic impurities such as acetylene, methyl acetylene, vinyl acetylene, ethyl acetylene, 2-methyl-1-buten-3-yne and the like, as well as diene compounds, such as butadiene, propadiene, and the like, in various crude mixed C2-C5 streams need to be removed with minimum loss of useful materials such as ethylene, propylene, butene, pentene, and the like in the feed streams. The preferred technique for the purification in commercial practice is the selective hydrogenation of acetylenic and diene compounds over hydrogenation catalysts.
Crude C5 olefin-containing streams may include various dienes and acetylenes, which often must be removed before use of the C5 olefin-containing stream in downstream processing units, such as a downstream metathesis unit. In addition to the need to remove dienes and acetylenes, which produce coke and shorten metathesis catalyst run length, cyclopentene must also be removed from the C5 feed to a very low level, such as less than 0.5 wt %, 1.5 wt %, or 2.5 wt %, as cyclopentene may undergo undesirable ring-opening metathesis polymerization in the downstream metathesis unit.
Various feeds may be used to provide the C5 olefins, including C5 fractions from crackers, such as a fluid catalytic cracker (FCC) or a steam cracker. The mixed C5's from such crackers is typically processed to result in feed of only the desired C5's, with minimal impurities, to the metathesis unit. For example, C5's from an FCC unit may be fed to a selective hydrogenation unit and fractionated to separate the C6+ hydrocarbons and cyclic C5 olefins from linear and iso C5 olefins, which may then be used in a metathesis process.
Such a simple system may not be suitable for steam cracker C5 products, however. Steam cracking processes produce C5 hydrocarbon streams having a very high concentration of cyclopentadiene and dicyclopentadiene, in addition to linear C5 dienes, isoolefins, and acetylenes, relative to FCC C5 products. The higher diene content, for example, if processed similar to an FCC C5 product, may result in high rates of catalyst fouling and potential runaway reactions. Further, sulfur compounds present in the C5 feed could potentially inhibit/damage the catalyst performance.
U.S. Pat. No. 3,492,220 to Lempert et al, 1970, disclosed a process for hydrotreating a full boiling-range pyrolysis gasoline containing styrene and C5 and lighter hydrocarbons with a sulfided nickel catalyst under conditions that produce either stable gasoline in a single zone or a substantially olefin-free, sulfur-free product in two or more reaction zones. However, this disclosed process is not selective to olefins.
U.S. Pat. No, 3,691,066 to Carruthers et al., 1972, disclosed a supported nickel catalyst for selective hydrogenation of unsaturated gasolines, e.g. steam cracker gasoline. The diene content is reduced from 4-55% wt to below 0.5% wt. Total sulphur content of the feedstock is 0.1-1.5% wt., of which 0.003-1.0% wt, may be thiophenic sulphur. Runs of over 500 hours, particularly over 1000 hours were noted are possible. It was stated therein that fresh wholly elemental nickel catalyst is not selective in its hydrogenation activity and will hydrogenate mono and diolefins and aromatics and the fact that monoolefins and aromatics remain unhydrogenated in the present process is due to the partial sulphiding of the nickel catalyst by the thiophenic sulphur normally present in the feedstock.
U.S. Pat. No. 4,059,504 to Bauer, 1977, disclosed a process in which pyrolysis gasoline is stabilized by hydrotreating in the presence of a catalyst of cobalt-tungsten sulfide supported on high surface area alumina. It was stated in this patent that the non-noble catalysts, the most widely used being Ni, W—Ni, Ni—Mo and Co—Mo, supported on a high-surface alumina base, require either pre-sulfidation or operation with high sulfur content feeds. It was also noted that the non-noble metal catalysts heretofore used in the art have the disadvantage in that they tend to produce polymers during the hydrotreating. In this patent, the active form of the catalyst is the sulfide form, and the catalyst is preferably pre-sulfided, although when using high sulfur feeds, the active sulfide form is produced on-stream, whereby, in some cases, pre-sulfiding is not required.
EP0011906 to Christy et al., 1983, disclosed a process for the selective hydrogenation of dienes in pyrolysis gasoline which includes catalytic hydrogenation of the pyrolysis gasoline in at least three consecutive reactors. In at least two of the consecutive reactors, the process includes recirculating part of the hydrocarbon mixture emerging from a reactor over that reactor. The catalyst used for the catalytic hydrogenation comprises partially sulfided nickel on alumina as a support. The weight ratio of hydrocarbon mixture recirculated to the first reactor and the pyrolysis gasoline fed thereto is from 5 to 15 and to the second reactor from 2 to 4. The examples provided show the unsaturate recovery (dienes+olefins) for the first case is 91.27% and for the second case is 87.79%, with 5000 dienes ppm remaining in the product stream.
U.S. Pat. No. 6,686,309 ('309) to Didillon et al, 2004, disclosed a palladium based catalyst with at least one metal selected from molybdenum and tungsten, in the form of at least one oxide, for selective hydrogenation of unsaturated diolefinic compounds in gasolines without hydrogenating the aromatic and mono-olefinic compounds. In the background of the '309 patent, it was acknowledged that two main types of catalyst are generally used for hydrogenating diolefins and styrenic compounds: catalysts using noble group VIII metals such as palladium, and those using non-noble group VIII metals such as nickel. It was stated that the second type of catalyst generally has a lower activity and undesired oligomerizing properties, which necessitates frequent regeneration and the use of a distillation column after hydrogenation to eliminate the heavy compounds. Further, such catalysts were noted as useful to only treat feeds containing large quantities of mercaptans, such as that found in catalytic cracking gasolines.
CN101254465 A (Sinopec) disclosed a selective hydrogenation catalyst for cracking C5 streams, which contains the following components in the given mass percentages: Ni 10-35%, La 0.5-3%, Ag 0.3-3% and aluminum oxide carrier 59-89.2%, and can contain other metals. Metal La and precious metal Ag are claimed to be required to improve catalyst selectivity and resistance to carbon deposition. It is stated that since cracking C5 fraction containing a lot of dienes is easy to generate a polymer which will cover the active sites of the catalyst and reduce catalyst activity, adding an alkaline or alkaline earth metal catalyst will be good to reduce the formation of polymers.